Below are my own suggestions/opinions in no particular order.

1) Use lots of reference
2) Use polygons
3) Add geometry as you need it. Don't start with a lot of geometry up front.
4) Practice a lot. Finish one, scrap it, and start the next one. Don't try to make a model "perfect". Learn from it and apply what you learn to the next.
5) Rinse and repeat.

Good luck.